Heavenly Bodies and Their Accidents

As we continue our weekly readings of Sulayman al-Mahri’s Mir’at al-salak (Mirror of Travellers; see our previous post), wondering about the relation between cosmography and geography and navigation, as in one of the recent RUTTER Blog posts, we have come across an interesting live example of “saving the phenomena”, straight from the 16th-century Arabian Peninsula.

The context is Ptolemaic through-and-through, with al-Mahrī explaining in Chapter 5 what relates to the “accidents” of the planets in their courses. He takes a while to describe the mechanics of planetary movement, dwelling on the details of retrogradation which were so fundamental to the whole Ptolemaic edifice. So we need to brush up conscientiously on our pre-modern astronomy terms in Arabic, the epicycles, deferents, with their apogees and perigees, to follow carefully the exposition. But then, after a thorough explanation, we come across the following,

Now, it is obvious that the heavenly bodies bring to completion the circular movement inside their orb without experiencing any alteration that touches on the orb. For their movement is neither faster nor slower, nor do they go backwards on their path, or halt their movement in the actual fact itself (nafs al-amr). But all this has to do with the relation between our vision (hasab ru’yatinā) and the composition of the movements (tarkīb al-harakāt).

While it has become commonplace to cite Aristotle on the matter of “saving the phenomena”, it is well known that from the earliest times of Greek philosophy there was a tension between the perfect motion of the spheres—cf. Timaeus, “time is the moving image of eternity”—and the perceived irregularity of the astronomical phenomena. Most notable and seminal are the Platonic passages in Timaeus, Epinomis and the Laws, but for example, and more directly astronomical, here is Geminus of Rhodes (1st century BC),

The Pythagoreans, who were the first to apply themselves to investigations of this kind, assumed the movements of the Sun, the Moon and the five planets to be circular and uniform. They would not admit, with reference to things divine and eternal, any disorder such as would make them move at one time more swiftly, at one time more slowly, and at another time stand still.

Ptolemy (Almagest III, 1 and XIII, 2) is very clear about preserving as much as possible the simplicity of the models (Gr. hypotheseis), and the Middle Ages saw countless back and forth arguments on this matter, which might be said to be a conversation, at times heated!, between cosmology and cosmography, what we know to be de iure, and what we perceive to be de facto. It is all a series of disputationes maybe, lively and truly philosophical. How remarkable, then, that an unassuming Yemeni pilot of the 16th century, man of praxis and adventure, should join the conversation, echoing in his clear, matter-of-fact discourse, the arguments of so many centuries before him—from Pythagoras to al-Mahrī, just a blink of a scientific eye! [JA]

“The Clouds” and Stellar Fraternity

Just a brief note this week, prompted by a passing remark by Louis Massignon in his article about the Magellanic Clouds, “Les nuages de Magellan et leur découverte par les arabes”. Apart from their astronomical interest, these two galaxies, called the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), are especially notable for being about 20 degrees from the South Celestial Pole, and as such of great aid to navigation. Even more, their current and historical position bears echoes of an even greater archaic importance, for around 1000BC they were almost on the Pole.

Unknown to Ptolemy, these Two Clouds (al-Sahābatān) were first mentioned in writing by al-Sūfī in the 10th century, and as such they were known to Ibn Mājid, and a common fixture of Indian Ocean navigation. The Celestial Pole can be located at the vertex of an equilateral triangle based on them (or alternatively a larger triangle based on Canopus and Achernar)—see the illustration below, where the Pole is approximately within the blue circle, and the two faint Clouds show on the centre right.

View from Mossel Bay, South Africa, 3 February 1488,
around the time that Bartolomeu Dias first rounded the Cape.

Massignon comments on the warmth of the relation of southern hemisphere nations to the Two Clouds, comparing it to the relation with the Pleiades, and observing: “it is fraternal respect rather than worship.”

The word “government”, we tend to forget, is not directly related to force and to the “might is right” delusion, or to other delusions of imperial grandeur, but to the Greek root kubernao, “to steer”, “to pilot” on the authority of knowledge. That we can benefit from our elder siblings in the sky, all those guiding lights, and to do so fraternally, like a passenger approached a pilot in the middle of the night on board a ship, to have a chat and learn… this is a reassuring thought in stormy times, and as good pilots know, some times are stormier than others. [JA]

How Many Do We Need?

We have once again come across a mention of al-Sufi’s Book on the Shapes of Stars (Kitāb suwar al-kawākib, 10th century), considered by many as the the most important update on Ptolemy’s enormously influential Almagest (originally entitled Μαθηματικὴ Σύνταξις; the Syntaxis Mathematica, 2nd century).

The fascinating history of this relation has been explored time and again from many angles. Briefly, as regards uranography—the “cataloguing of stars” contained in books 7 and 8 of the Almagest—al-Sufi was the pivotal figure who revised the Alexandrian material and prepared it for further developments, culminating with Tycho Brahe’s Rudolphine Tables (1627).

Now, coming to our nautical context: when Ibn Mājid mentions al-Sufi, he invariably adds “and his forty-eight constellations,” a number which seems to go back to Ptolemy. Studying the lunar mansions has made palpable how arbitrary the division of the ecliptic is: you have a rotating circle dotted with shiny specks, you introduce some boundaries and project your lines. You can have 28 mansions or 12 signs or 36 decans or even 144 dodecatemoria. The division in 36 is easy to understand if we think of the rationale of the Babylonian “Three Stars Each” catalogues: for each twelfth of the ecliptic you select one equatorial, one southern and one northern star or asterism, so you cover a wider area and thus give more parameters for orientation (we are not going to waste all those stars!).

In a very useful summary of Ibn Mājid’s uranography, Ibrahim Khoury speaks of 24 major stars used in Indian Ocean navigation. Why 24? and why 48? These figures might be a Greek alphabetic reminiscence: the Greek alphabet has 24 letters, and using this number meant you were speaking of “the alpha and omega” of the stars, the “heavenly ABC.” It meant it was enough, that no other element was needed. I suspect some earlier source for this, perhaps Hipparchus… but the abiding and concrete question is how many stars do we really need to find our way. Right now, dear reader, when it is dark, how many stars in the sky do you need to know in order to find your way home? [JA]